1. Field of the Invention
This invention relates generally to VTOL aircraft; and more particularly to tiltrotor aircraft for operation in the intermediate-speed range near 400 knots.
2. Prior Art
This section will review prior work in two fields that have not been interrelated heretofore:
(a) the history and current problems of tiltrotor aircraft; and
(b) jointed-wing aircraft theory and test results.
(a) CANTILEVER TILTROTORS--Tiltrotor aircraft which have flown successfully include the little-known 1954 Transcendental Model 1G. Lambermont, P.; and Pirie, A., Helicopters and Autogyros of the World, Second Ed., A. S. Barnes and Co., Cranbury, N.J. 08512 (1970).
Other such craft include the Bell XV-3, which first flew in 1955, and the Bell XV-15. At the time of writing, the Bell-Boeing V-22 Osprey and the Bell-Boeing Pointer RPV are nearing their first flights. All these craft alike are beset by five major problems, which will be set forth shortly.
The XV-3 and Model 1G employed tilting rotor shafts, the powerplant being fixed within the fuselage. The Pointer uses a similar arrangement, but has nacelle-like fairings around the rotor shafts.
The XV-15 and V-22 instead have powerplants mounted in the nacelles to tilt with the rotor. It is also possible to have nontilting powerplants: FIGS. 19 and 20 show a powerplant in a fixed wingtip nacelle driving a tilting shaft.
According to Aviation Week, the United States Government has ordered 688 V-22s--for a total cost of $23.7 billion. Shifrin, C. A., in "Bell/Boeing Team Rolls Out V-22 Osprey Tiltrotor Prototype," 1988 Aviation Week 19-20 (May 30).
This is an immense investment, which may influence subsequent tiltrotor designs (both civil and military) to copy certain features of the V-22. Therefore in the analysis that shortly follows I shall pay special attention to some specific problems of the V-22.
Meanwhile, the strength of interest in civil applications has been noted in Kocks, K., "A U.S. Civil Tiltrotor: Is the Gauntlet Thrown?", 1987 Rotor and Wing International 30-31 (June). This article states that European manufactures hve organized a five-nation program, called EUROFAR, to produce a competitor to a U.S. civil derivative of the V-22.
The same reference describes a current FAA/NASA/DOD study. Kocks observes that that study "already indicates that a tiltrotor can offer significant airport congestion relief and do it economically under a realistic scenario."
Problem 1: Thick Airfoils--The main advantage of the tiltrotor over the helicopter is speed. Jane's All the World's Aircraft, 1986-87 Edition, gives the XV-15 maximum level speed at 17,000 feet altitude as 332 knots (Mach 0.53). Tilt-fold rotors offer the potential for still higher cruising speeds.
A limiting factor, however, on realizing such high speeds is airfoil thickness, and consequent low drag divergence Mach numbers. Current tiltrotor aircraft must employ very thick airfoils, to obtain adequate wing stiffness for support of the rotors--and, where applicable, the powerplants.
The V-22 wing airfoil has a thickness/chord ratio of twenty-three percent--limiting practical operational speeds to less than 300 knots. Such thick airfoils prevent the tilt-rotor concept from achieving its high-speed potential.
For example, Johnson, Lau and Bowles show that at 400 knots V-22 wing and rotor compressibility effects would each absorb approximately ten percent of the total power. Johnson, W.; Lau, B. H.; and Bowles, J. V., "Calculated Performance, Stability, and Maneuverability of High Speed Tilting Proprotor Aircraft," 11 Vertica 317-39 (1987).
Using thinner airfoils at no change in weight (which is impossible in the prior art) would eliminate compressibility drag at 400 knots, reducing cruise power by approximately ten percent. Furthermore, eliminating wing compressibility drag with no structural weight penalty would save fuel.
Reduction in fuel weight for a given mission reduces hover thrust and power. This leads to reduced engine and transmission weight.
Thus the compounded effect of eliminating wing compressibility drag would be large. Johnson, Lau and Bowles state that it could lead to a reduction in gross weight of ten percent.
Problem 2: Hover Thrust Losses--Felker and Light show that the net hover thrust of tiltrotor aircraft is typically eleven percent less than the isolated thrust of the rotors at the given shaft power. Felker, F. F.; and Light, J. S., "Aerodynamic Interactions between a Wing and a Rotor in Hover," 1988 J. American Helicopter Society 53-61 (April).
This thrust loss seriously degrades the load-carrying capability of the vehicle. It is caused by two phenomena, as illustrated in FIG. 21.
One is the direct drag (download) of the wings. The other is the recirculation fountain effect occurring near the aircraft plane of symmetry.
Problem 3: Aeroelastic Wing/Rotor Coupling--Aeroelastic problems of tiltrotor design are well known. Pylon/rotor whirl flutter avoidance requires careful tailoring of wing structural mode frequencies.
Attempts to resolve this problem by strengthening or otherwise stiffening the wing tend to aggrave the airfoil-thickness problem discussed above, as long as the wing is a cantilever type.
Problem 4: Tail Vibration--As described by Bilger, Marr and Zahedi, the horizontal and vertical tail assembly of the XV-15 experience severe oscillatory loads. At low speeds (twenty to fifty knots) the rotor tip vortices impact the tail, producing what those authors describe as a random "whipping" impulsive loading. Bilger, J. M.; Marr, R. L.; and Zahedi, A., "In-Flight Structural Dynamic Characteristics of the XV-15 Tilt-Rotor Research Aircraft," 19 J. Aircraft 1005-11 (November 1982).
Above fifty knots, with nacelle tilt angles greater than seventy degrees to the flight path, the effect of the rotor wake at the tail is primarily blade passage (three per revolution). At 110 to 130 knots the dominant tail excitation comes from the vortex shed from the wingtip/nacelle junction.
At still higher speeds the oscillatory loading stems primarily from normal wing wake turbulence. All these variegated loading geometries--arising from varied rotor orientations that must be accepted as normal in a tiltrotor craft--once again call for structural reinforcements that are undesirable in terms of weight or drag penalty, or both.
Problem 5: Rotor-Wing Interference in Cruise--Current tiltrotors employ thick, large-chord wings having leading edges located only a short distance aft of the rotor (typically 0.25 times rotor radius). Thus each blade cycles through the wing upwash field at one per revolution.
This reduces propulsive efficiency and increases vibratory loads on the wing. The problem could be alleviated by moving the rotor further ahead of the wing, but this would require increasing rotor mast height (i.e., driveshaft length). For cantilever wings, such mast-height increase is undesirable because it tends to reduce the speed at which whirl flutter will occur.
Another drawback of short mast height, noted by Felker and Light, is that it increases the wing download in hover. Felker, F. F.; and Light, J. S., "Rotor/Wing Aerodynamic Interactions in Hover," NASA TM 88255 (May 1986; also presented at 1986 AHS Annual Forum).
They present test data showing that with a rotor located 0.217 times rotor radius R above a model V-22 wing the download was 0.14 times the rotor thrust T, whereas for greater mast height of 0.655R the net wing download was only 0.11T.
Felker and Light point out that this trend is not predicted by steady wake and slipstream contraction considerations. They ascribe it to large periodic airloads at the blade passage frequency.
Therefore short mast height also aggravates the vibration induced by such periodic loads. This provides another incentive to consider increasing mast height, but again whirl flutter would then set in at lower speed, thus limiting overall speed of the aircraft.
(b) JOINED-WING THEORY AND TESTS--The joined-wing airplane is a known type of aircraft configuration employing two sets of wings rigidly connected together to form a triangulated self-bracing structure.
Finite-element structural analyses and wind-tunnel tests have shown that, compared to cantilever-wing aircraft, joined-wing aircraft are lighter, stiffer, and have higher span-efficiency factors, giving lower induced drag. The joined wing also permits thinner airfoils to be used, thus increasing the Mach number for drag divergence and the maximum speed.
I have presented a complete survey of the joined wing in my paper, "The Joined Wing: An Overview," 23 J. Aircraft 161-78 (March 1986). Also informative are U.S. Pat. Nos. 3,942,747, 4,053,125, and 4,365,773. Therefore only the points of special interest to the present invention are listed here.
It has been shown that, compared to a conventional wing-plus-tail of the same span and total area made from the same material and carrying the same load, a joined wing can have these advantages:
(1) lighter by as much as forty-two percent; PA0 (2) stiffer--for example, a twenty-six-percent increase in flutter speed; PA0 (3) thinner-airfoil suitability, with less weight penalty; PA0 (4) more span-efficient--for example, nine percent less induced drag; and PA0 (5) better area-ruling, giving less transonic drag rise.
Eight different wind-tunnel models have been tested since 1979. No fundamental stability or control deficiencies have been found in any of those models. Limitations of the joined wing relate primarily to its novelty and design complexity.
For example, the small folded area (spotting factor) required for some shipboard deployments may demand complex wing-folding mechanisms. In addition, joined-wing craft are subject to Reynolds number reduction due to shorter wing chords.
Heretofore no relationship beween tiltrotor aircraft and joined-wing theory has been proposed or even suggested.